Hitherto, there is disclosed an inner-wadding type writing instrument wherein an ink occlusion body is impregnated with a reversibly thermochromic aqueous ink composition capable of forming handwriting whose color tone changes depending on a change in temperature and is received in an axle body (see, e.g., Patent Document 1).
The above reversibly thermochromic aqueous ink composition solves a problem that a reversibly thermochromic microcapsule pigment is gradually flocculated and precipitated in the ink occlusion body owing to a difference in specific gravity between the reversibly thermochromic microcapsule pigment and a vehicle and hence handwriting is lightened or deepened in color depending on the state where a pen body is downward (an inverted state) or upward (an erecting state). Specifically, it solves the problem by incorporating a water-soluble polymer flocculant into the ink to make the microcapsule pigment suspended in a loose flocculated state through a loose crosslinking action of the polymer flocculant.
However, although it is possible to suppress the flocculation and sedimentation of the microcapsule pigment for a short period of time by incorporating the polymer flocculant, it is difficult to completely suppress the lightening and deepening in color of handwriting attributable to the difference in specific gravity between the microcapsule pigment and the vehicle. Particularly, handwriting is sometimes lightened with time in an erecting state or handwriting is sometimes lightened by the application of vibration at transportation or at the time when it is carried on in an erecting state.